1. Field of the Invention
The present invention relates to a pneumatic tire and more particularly to a pneumatic tire having a tread with a block pattern.
2. Description of the Related Art
Sipes formed in ribs and blocks of a tread exhibit the effect of cutting through water films on road surfaces, similar to edges of grooves. The sipes also facilitate deformation of ribs and blocks and help exhibition of rubber hysteresis loss. Therefore, sipes are widely used in studless tires.
As shown in FIG. 3, studless tires generally have zigzag or straight sipes 100, extending substantially parallel to tire circumferential-direction end portions of blocks (i.e., the end portions of blocks which are continuous with the block end portions running along the circumferential direction of the tire). Alternatively, as shown in FIG. 26, studless tires generally have sipes 102 which are approximately perpendicular to the circumferential direction of the tire in each block 101.
However, when the sipes incline in the same direction on each block as described above, and when tires are used on snowy or icy roads, the effects of the sipe is satisfactory only for driving direction. Moreover, the degree of deformation of the blocks during driving is markedly different depending on the direction of the applied force. This means that the conventional method of forming sipes is not effective for improving cornering property, although the method is effective for improving braking and traction properties on ice. Moreover, when the tires are used on dry or wet roads, the rigidity of the blocks with respect to forward or backward force is small. Therefore, controllability at small steering angles is often insufficient. Even if the angle of sipes with respect to tire circumferential direction is changed, it is difficult to obtain suitable properties for both of braking and traction because rigidity of the block with respect to the forward force is different from that with respect to the backward force.
To improve tire braking on icy roads, it is desirable to increase sipe density at the central portion of the blocks. However, when the density of the sipes in an entire block is increased, block rigidity decreases and there is the possibility that controllability for dry and wet roads, and abrasion resistance of the tire are adversely affected.
When the density of sipes on entire blocks is excessively increased, the area of the tread contacting road surfaces decreases due to blocks collapsing and the properties when driving on icy roads also deteriorate. Moreover, in the above case, defects such as bareness and chipping tend to be more frequent during tire production.
For simultaneous good block rigidity and sipe effect (edge effect and water removal), it is desirable that the density of sipes be increased in the central portion of a block and decreased in block peripheral portions (for example, Japanese Patent Application Laid-Open No. 9(1997)-164816). However, from the standpoint of production, it is difficult to satisfactorily produce such a tire using conventional technology.
An object of the present invention is to overcome the above problems of conventional methods of forming sipes and provide a pneumatic tire having improved properties for icy and snowy roads, and in particular, good properties when used on icy roads.
Another object of the present invention is to provide a pneumatic tire having a block pattern with suitable sipe effect in multiple directions, good block rigidity with respect to force in any direction, and high density of sipes in the central area of each block.
Still another object of the present invention is to provide a pneumatic tire having a block pattern with suitable sipe effect in multiple directions, good block rigidity with respect to force in any direction, high density of sipes, and good tire properties for both braking and traction.
Still another object of the present invention is to provide a pneumatic tire having a block pattern with satisfactory sipe effect in multiple directions, good block rigidity with respect to force in any direction, and good controllability and resistance to uneven wear.
Still another object of the present invention is to provide a pneumatic tire having a block pattern which provides improved properties when driving on icy and snowy roads without adverse effects to properties when driving on dry roads and wet roads.
As the first aspect, the present invention provides a pneumatic tire comprising a tread having blocks defined by a plurality of intersecting grooves, wherein each of the blocks have at least one pair of sipes which are formed from first and second sipes, the first sipe extends from one of two tire axial direction end portions of the block and is inclined with respect to a circumferential direction of the tire. The second sipe extends from the other tire axial direction end portion of the block and is inclined with respect to the circumferential direction of the tire. The first and second sipes are alternately disposed along the circumferential direction of the tire. An end portion of the first sipe in the vicinity of a center of the block, and an end portion of the second sipe in a vicinity of a center of the block, intersect an imaginary common line extending along the circumferential direction of the tire. Further, the first and second sipes are inclined in opposite directions.
In the block, the sipe edge effect can be obtained for force applied from any direction since the first and second sipes are disposed alternately and the first and second sipes are inclined in opposite directions. For example, when a force from a direction parallel to the first sipe is applied to the tire, the second sipe provides the edge effect, although there is substantially no sipe edge effect from the first sipe in this case. That the first and second sipes incline in opposite directions means that, in the portion of the tire contacting the road surface when the tire is used on a vehicle, the first sipe is inclined in a manner such that the end portion of the first sipe, which end portion is at the side of the center of the block is placed in the front or in the rear of the other end portion, the second sipe is inclined in a manner such that the end portion of the second sipe, which end portion is at the side of the center of the block is placed in the front or in the rear of the other end portion. The angle between the first sipe and the circumferential direction of the tire is not always the same as the angle between the second sipe and the circumferential direction of the tire. Therefore, the cornering property when driving on icy road, on which the sipe edge effect is particularly effective, can be improved.
Since the end portion of the first sipe which is at the side of the center of each block and end portion of the second sipe which is at the side of the center of each block intersect an imaginary common line extending along the circumferential direction of the tire, the number of sipes in the central area of the block is greater relative to that in the peripheral areas of the block (end portions of the block).
Since the density of sipes increases in the central area of the block in which, when driving on icy roads, water film forms more easily than in the peripheral areas of the block, the number of edge components is effectively increased, and water film absorption properties are enhanced, and braking and traction properties improve. Here, edge components designate portions of both sides of a groove and portions of both sides of a sipe which are substantially straight and which form an edge when the sipe is open.
Moreover, density of sipes in the peripheral areas of the block (end portions of the block) can be lower than density of sipes, formed by conventional methods, in the peripheral areas for providing sufficient block rigidity. More specifically, since rigidity at the central area of the block can be decreased while maintaining sufficient block rigidity in peripheral portions, a sufficient area of the portion contacting the road surfaces can be obtained by suppressing falling down (collapsing) of the entire block during driving, and the controllability when driving on dry and on wet roads can be improved compared to that of conventional tires.
In the above tire, it is preferable that each block has pairs of sipes formed of first and second sipes. The first sipes on each block are parallel with one another, as are the second sipes on each block.
Accordingly, block rigidity can be properly maintained, and uneven wear can be reduced.
Further, in the above tire, it is preferable that the blocks are defined by grooves extending along a circumferential direction of the tire and grooves which intersect the circumferential grooves. The first sipes of each block disposed linearly along the circumferential direction of the tire are all inclined in the same direction. Further, the sipes are disposed at a predetermined interval. The second sipes of each block disposed linearly along the circumferential direction of the tire are all inclined in the same direction and also disposed at a predetermined interval.
By disposing the first or second sipes at a constant interval in each block, change in rigidity of the central area and the peripheral areas of the block depending on the direction of the applied force can be suppressed, along with uneven wear.
Moreover, by forming sipes in a manner such that first sipes or the second sipes formed in blocks which are disposed in a straight line along the circumferential direction of the tire are disposed in the same direction and such that a distance between the first sipes or the second sipes formed in each block is constant, and by repeating in the circumferential direction of the tire a specified block pattern, which has sipes, along the axial direction of the tire, the tire abrades more uniformity.
In the above tire, it is preferable that the grooves extending along the circumferential direction of the tire comprise first grooves and second grooves which are narrower than the first grooves, and the second grooves are disposed between the first grooves and have a width of 0.5 to 6 mm.
Even when the blocks divided by the first grooves are large, a decrease in the ability of the central area of the block to remove water can be prevented since the second groove is disposed dividing each of the blocks into two portions. By disposing the second grooves, the number of edge components extending in the circumferential direction increase and cornering properties on ice and on snow improve. When the width of the second groove is smaller than 0.5 mm, there is increased risk that the ability of the blocks divided by the first grooves and the second grooves for removing water will deteriorate. When the width of the second groove exceeds 6 mm, the divided blocks become small and rigidity of the block decreases, although the ability of the blocks to remove water improves.
It is preferable that the first sipes and the second sipes do not intersect each other. When the first sipe and the second sipe intersect each other and the first sipes intersect each other and the second sipes intersect each other, rigidity substantially decreases at the portion of the intersection and uneven wear may take place. When no sipes intersect each other, this problem can be substantially avoided.
It is preferable that the first and second sipes in the blocks disposed in the central area of the tread are disposed at an angle of 45 to 85xc2x0 and more preferably 55 to 80xc2x0 relative to the circumferential direction of the tire. When the angle between the first (or second) sipe in the block disposed in the central area of the tread and the circumferential direction of the tire is smaller than 45xc2x0, the number of edge components effectively working in the circumferential direction of the tire is insufficient and traction and braking properties deteriorate. When the angle between the first (or second) sipe in the block disposed in the central area of the tread and the circumferential direction of the tire exceeds 85xc2x0, the angle between one of the first sipe and the second sipe and an extended line of the other nears 180xc2x0 and the advantageous effect of improving the cornering properties decreases.
It is preferable that an angle between one of first and second sipes and an extended line of the other is 10 to 45xc2x0. Thereby properties on snow (in particular cornering properties), controllability on dry and wet road surfaces under application of force in a small steering angle and braking properties on ice, in particular, on smooth and slippery road surfaces are improved.
In the above pneumatic tire, it is preferable that the distance between the first sipes and that between the second sipes are 2 to 5 mm. These distances are the optimum between the sipes to obtain the effect of the sipes described above.
A central area C and two end areas E are obtained by imaginarily dividing each block into three equal parts. xcfx89c represents a total length, in the circumferential direction of the tire, of respective portions of one first sipe and one second sipe which portions are located within the central area C. xcex4c represents a total length, in the axial direction of the tire, of those. xcfx89e represents the longer of a length, in the circumferential direction of the tire, of a portion of one first sipe, which portion is located in an end area E, and a length, in the circumferential direction of the tire, of a portion of one second sipe, which portion is located in an end area E. xcex4e represents the longer of a length, in the axial direction of the tire, of the portion of one first sipe, which portion is located in the end area E, and a length, in the axial direction of the tire, of the portion of one second sipe, which portion is located in the end area E. It is preferable that ace xcfx89c, xcfx89e, xcex4c, and xcex4e satisfy the relations: xcfx89c greater than xcfx89e and xcex4c greater than xcex4e. When the above relations are satisfied, a greater density of sipes can be achieved in the central area of the block and the above described effects can be obtained.
In the above pneumatic tire, it is preferable that the depths of end portions of the first or second sipe are shallower than that of a central portion thereof Thus, the rigity of the end portions of the block is higher. Further, although the density of sipes is greater at the central portion of the block than at the end portions thereof, the rigidity at the central portion of the block is sufficient.
It is preferable that blocks disposed in side areas of the tread have sipes whose angle with respect to a circumferential direction of the tire is equal to or greater than an angle between each of the sipes formed in blocks disposed in the central area of the tread and the circumferential direction of the tire.
By forming sipes in a manner such that the angle between each of the sipes formed in the blocks disposed in the side areas of the tread, i.e., in both shoulder portions, and the circumferential direction of the tire is equal to or greater than the angle between each of the sipes formed in the blocks disposed in the central area of the tread and the circumferential direction of the tire, i.e., by disposing the sipes in side areas of the tread in the direction closer to the axial direction of the tire, uneven wear which tends to occur in the shoulder portions can be suppressed. It is preferable that the angle between the circumferential direction of the tire and the sipes, which extend from the end portion in the vicinity of the shoulder of the blocks in the area of both sides of the tread, is greater than the angle between the circumferential direction of the tire and the sipes which are formed in blocks in the central area of the tread.
The central area and the side areas of the tread described above are the central area and the side areas, respectively, obtained by dividing the tread into three areas in the axial direction.
It is preferable that a distance between end portion of the first sipe, which end portion is in the vicinity of the center of the block, and end portion of the second sipe, which end portion is in the vicinity of the center of the block, in an axial direction of the tire is 2 to 15 mm and more preferably 3 to 10 mm. When the distance is less than 2 mm, the density of sipes in the central area of the block cannot be increased. Even when the tire has a large size, i.e., even when the tire has large blocks, it is preferable that the distance is 15 mm or smaller. The reason is as follows. When the distance is excessively great, the distance between the sipes in the circumferential direction of the tire must be made greater in order to dispose the first sipes and the second sipes in a manner so that the first sipes and the second sipes incline in opposite directions and, at the same time the first sipes and the second sipes do not intersect each other or do not connect to each other. However, when the distance between the central side end portions of the first sipes and the second sipes in the axial direction of the tire is made greater, the number of the sipes formed in the block decreases and density of sipes in the central area of the block cannot be made great. To prevent the decrease in the number of the sipes, it is necessary that the angle of the sipes with respect to the circumferential direction of the tire be made close to 90xc2x0, i.e., the direction of the sipes be made closer to the axial direction of the tire. When the direction of the sipes is made closer to the axial direction of the tire, cornering properties on ice deteriorates. Therefore, it is preferable that the distance between the central side end portions of the first sipes and the second sipes in the axial direction of the tire be 15 mm or less.
An area of the tread having 30 to 70% of an entire width of an area contacting road surfaces at the center of the tread, is referred to as a central area. Remaining areas are referred to as side areas. It is preferable that a chevron formed by one of the first and second sipes and an extended line of the other sipe of the pair in blocks in the central area, and a chevron formed by one of the first and second sipes forming a pair and an extended line of the other sipe of the pair in blocks in the side areas are placed in directions opposite to each other with respect to the circumferential direction of the tire. Improved sipe edge effects can be obtained for both braking and traction. The width of the area contacting road surfaces described above is the width of the area of the tire contacting the road surface when the tire is attached to a rim suitable for the tire size in accordance with standards of various countries such as the standard of Japanese Automobile Tire Manufacturers Association in Japan, the standard of ETRTO (European Tire and Rim Technical Organization) in Europe and the standard of Tire and Rim Association in the United States of America, with an applied load of 88% of the maximum allowable.
It is preferable that the blocks have pairs of sipes which are formed from first and second sipes. When each block is equally divided (imaginarily) in first and second portions by a line extending along the tire axial direction, apices of chevrons each formed by one of the first sipe and the second sipe forming a pair in the first portion and an extended line of the other sipe of the pair face apices of chevrons each formed by one of the first sipe and the second sipe forming a pair in the second portion and an extended line of the other sipe of the pair. Thereby, the sipe effect is present even when force is applied in various or multiple directions, and improved edge effects are present for both braking and traction in each block.
It is preferable that in each block, an angle between the radial direction of the tire and a first wall of the block at a side of an apex of a chevron formed by one of the first and second sipes forming a pair, and an extended line of the other sipe of the pair is greater than an angle between the radial direction of the tire and a second side wall of the block at a side opposite to said first side wall. Thereby, rigidity of each block is sufficient for both braking and traction.
In the above tire, the central portions of first sipes and the second sipes can be zigzag or wavy and end portions can be substantially straight.
By making the central portion of the sipes zigzag or wavy, the total length of the sipe edge increases and water film on icy roads can be absorbed more efficiently. Further, because the number of edge components effective for the circumferential direction of the tire and the number of edge components effective for the axial direction of the tire can be increased, cornering performance in particular is improved.
By making the end portion of the sipe, which end portion is in the vicinity of the end of the block, substantially straight, the angle between the end portion of the sipe and the edge of the block can be kept substantially the same for the respective sipes and uneven wear can be suppressed. By making the end portion of the sipe, which end portion is at the central side of the block, substantially straight, the sipe opens more easily than a sipe which is formed in a zigzag or wavy pattern all the way from one end of the sipe to the other. Snow penetrates into open sipes, i.e., the sipes hold snow, and tire properties on snow are improved.
In the above pneumatic tire, it is preferable that the first and second sipes are one of curved and bent in the direction of the depth of the block and inclined with respect to a radial direction of the tire. Falling down (collapse) of blocks between the sipes is better suppressed by this structure when a force is applied thereto and properties on ice and on snow improve.
In the above pneumatic tire, it is preferable that a number of the first and second sipes formed on each block is two or more, and at least four sipes of the first sipes and the second sipes are one of curved and bent in the direction of the depth of the block and inclined with respect to a radial direction of the tire. Accordingly, when force is applied, portions of both sides of the sipes of the blocks support one another and falling down (collapse) of the blocks is prevented.
In the above pneumatic tire, it is preferable that rubber used in the tread is an expanded rubber. It is preferable that a degree of expansion of the expanded rubber is 10 to 50%. By combining the sipes and the expanded rubber, properties on snow and ice can be improved.
In the second aspect, the present invention provides a pneumatic tire which comprises a tread formed in a toroidal shape, a pair of side walls disposed at inner positions in a radial direction of the tire from end portions of the tread, and beads connected to the side walls at inner positions in a radial direction of the tire, wherein
the tread has a block pattern having pairs of blocks,
each pair includes a first block and a second block which are arranged in an axial direction of the tire in a chevron-like arrangement with an apex of the chevron-like arrangement directed in the tire circumferential direction, a tire circumferential direction end of the block pair at an apex side of the chevron-like arrangement of the block pair being a first tire circumferential direction end of the block pair, and a tire circumferential direction end of the block pair at a side opposite to the apex side being a second tire circumferential direction end of the block pair;
a side wall of the first block, which side wall is opposite a second block side of said first block, is a first side wall, a tire radial direction lower portion of the first side wall at the first tire circumferential direction end of the block pair being disposed further away from the second block than a tire radial direction upper portion of the first side wall at the first tire circumferential direction end of the block pair, the tire radial direction lower portion of the first side wall being gradually inclined toward the second block along the tire circumferential direction from the first tire circumferential direction end of the block pair toward the second tire circumferential direction end of the block pair,
a side wall of the second block, which side wall is opposite a first block side of said second block, is a second side wall, a tire radial direction lower portion of the second side wall at the first tire circumferential direction end of the block pair being disposed further away from the first block than a tire radial direction upper portion of the second side wall at the first tire circumferential direction end of the block pair, the tire radial direction lower portion of the second side wall being gradually inclined toward the first block along the tire circumferential direction from the first tire circumferential direction end of the block pair toward the second tire circumferential direction end of the block pair.
By forming pairs of blocks as described above, properties on ice and on snow can be further improved due to the blocks deforming advantageously when force in the circumferential direction of the tire is applied thereto due to an increase in the projected edge (i.e., W in FIG. 17). Deformation of the blocks to force applied along the axial direction of the tire or forces applied along directions other than that described above can be suppressed. Thus, properties on ice and on snow can be improved without sacrificing the properties on dry or wet road surfaces.
In the above pneumatic tire, it is preferable that the first block and the second block have a plurality of sipes which are disposed substantially parallel with end portions of the blocks to which end portions the apexes of the chevrons point.
In the above pneumatic tire, it is preferable that the angle between the first portion of the first side wall and the radial direction of the tire and the angle between the second portion of the second side wall and the radial direction of the tire are each +5 to +15xc2x0. When force is applied from the direction of the apex of the chevron, sufficient rigidity is obtained and falling down (collapse) of the block can be suppressed.
In the above pneumatic tire, it is preferable that an angle formed by the radial direction of the tire and the first side wall at the second tire circumferential direction end of the block pair and an angle formed by the radial direction of the tire and the second side wall at the second tire circumferential direction end of the block pair are xe2x88x925 to +2xc2x0. The side opposite to the side of the apex of the chevron tends to deform when force is applied thereto. Thus, properties on ice and on snow are improved.
In the above pneumatic tire, it is preferable that the distance between blocks of each pair is 0.3 to 1.0 mm. In this condition, blocks of each pair can support each other and deformation by the applied force from the axial direction of the tire decreases.
It is preferable that the rubber used in the tread is an expanded rubber having a degree of expansion of 10 to 50%. By the combination of the expanded rubber and the sipes in accordance with the present aspect, the properties on ice and on snow can be further improved.